Atmospheric furnace apparatus

ABSTRACT

A gas atmospheric furnace is provided which is used for carburizing or carbonitriding treatment of steel articles. A gas inlet for introducing an atmosphere gas into the furnace communicates with a passageway formed around the shaft of a fan which is used to agitate the gas. A different kind of gas is also introduced into the gas inlet through the same passageway. Another gas inlet is disposed in opposing relationship with the first mentioned gas inlet, and gases supplied through both gas inlets serve to maintain the temperature within the furnace constant.

United States Patent [191 Namba 1 Nov. 26, 1974 i 1 ATMOSPHERIC FURNACEAPPARATUS [75] Inventor: Keishichi Namba, Yokohama, Japan [73] Assignee:Tokyo Netsushori Kogyo Kabushiki Kaisha, Kanagawa-ken, Japan [22] Filed:Apr. 9, 1973 [2]] Appl. No.: 349,323

[30] Foreign Application Priority Data Apr, 7, 1972 Japan 47-34917 [52]US. Cl 432/203, 432/19, 432/26,

' 432/205 [51] Int. Cl. F27b 1/08, F27b 3/02 [58] Field of Search..'432/19, 26, 198, 200, 201, 432/202, 203, 205

[56] References Cited UNITED STATES PATENTS 2,039,429 5/1936 Lydon432/203 X 2,152,154 2,283,982 5/1942 Germany 432/202 X 2,869,848 1/1959Dickie et a1. 432/198 Primary Examiner-John .1. Camby Attorney, Agent,or Firm-Robert E. Burns; Emmanuel J. Lobato; Brucel Adams v [57]ABSTRACT A gas atmospheric furnace is provided which is used forcarburizing or carbonitriding treatment of steel articles. A gas inletfor introducing an atmosphere gas into the furnace communicates with apassageway formed around theshaft of a fan which is used to agitate thegas. A different kind of gas is also introduced into the gas inletthrough the same passageway. Another gas inlet is disposed in opposingrelationship vwith the first mentioned gas inlet, and gases suppliedthrough both gas inlets serve to maintain the temperature within thefurnace constant.

5 Claims, 3 Drawing Figures 3/1939 Robiette 34/36 X PATENIE HUV 26 1974SHEET 10F 2 FIG.- I.

PATENIE rmvzslsn SHEET 2 BF 2 FIG. 2

ATMOSPHERIC FURNACE APPARATUS The invention relates to a gasatmosphericfurnace and more particularly to a heat treatment furnacehaving improved gas inlets. Gas carburizing processes and gascarbonitriding processes are now well known in the art of case hardeningof steel articles. In the long known gas carburizing process, acarburizing gas including natural gas or propane gas is fedinto a heattreatment furnace which is hermetically sealed for carburizing anarticle to be treated under heat within the atmosphere. Gascarbonitriding process invloves heating in a gas atmosphere whichcomprises the carburizing gas to which is added ammonia gas, and hasbeen developed recently as demonstrated by the disclosure in BritishPat. No. 1,034,157 published June 29, 1966. Surface hardening achievedby carburizing is a transformation hardening, while the nitridinghardening is attributable to the formation of a compound and it isrecognized that the thickness of a hardened layer has a closerelationship with the nature of gas used as well as the heatingtemperature. For this reason, proper maintenance of the furnaceatmosphere and the furnace temperature is of primary importance.

Batch type furnaces have been used with the gas carburizing and gascarbonitriding treatments, and recently a continuous furnace also findsits application in these treatments. These furnaces generally include aheating element disposed therein along the opposite side walls and thebase, with a fan mounted on the celling. An atmosphere gas is introducedin a horizontal direction toward, the blades of the fan through thefur-- nace wall. Thus the fan stirs the atmosphere gas supplied throughthe gas inlet while the resulting forced circulation enables heat fromthe heating element to be conveyed throughoutthe space within thefurnace.

However, it is seen that the relative position of the fan and the gasinlet is such that there exists a location within the furnace whichalways remains under the influence of cold gas, thereby precluding auniform temperature distribution throughout the furnace from beingachieved. While fluid resistance governed by the furnace structure isinvolved with this problem, it is believed that the major difficulty iscaused by the directional flow of the atmosphere gas produced byrotation of the fan.

A similar drawback of unevenness in heating is found in attempting tomix different gases within the furnace, as may be required, for example,when mixing the car burizing gas with ammonia gas in the carbonitridingprocess. Because ammonium carbonate, (NH CO which is formed uponreaction of carbonic acid gas (CO contained in the carburizing gas withammonia (NH;,), is in the form of White crystals, below 58C, both gasescannot be mixed under normal temperature conditions before being fedinto the furnace.

Therefore, it is an object of the invention to provide an atmosphericfurnace having improved gas inlets capable of feeding an atmosphere gastoward a stirring fan in a manner which avoids a directional flow of thegas.

It is another object of the invention to provide an atmospheric furnacehaving a first gas inlet for feeding an atmosphere gas toward a fan anda second gas inlet located in opposing relationship with the first gasinlet for directing another atmospere gas, the nature of which is thesame as that of the first mentioned gas, to be merged with the firstmentioned gas diffused by the fan, thereby improving the thermal balancewithin the furnace.

' It is a further object of the invention toprovide an atmosphericfurnace having a common gas inlet for dif-' ferent gases which iseffective to effect an even mixing of the gases within the furnace.

In accordance with the invention, the atmospheric furnace comprises agas inlet which opens into the furnace from a passageway formed alongand around the shaft of a fan. The passageway communicated with a sourceof gas through a guide pipe. Where more than one gas is introduced intothe furnace, separatev guide pipes are provided to conduct therespective gases independently and communicate with the passageway. Theatmosphere gas fed into the furnace through the gas inlet if diffused asthe fan rotates, thereby preventing a directional flow. The fan and itsshaft are directly exposed to the flow of the atmosphere gas directedfrom the guide pipe into the passageway, but the gas in this area isunder normal temperature and hence provides a cooling function, which ishighly effective to prevent corrosion of these parts by hot atmospheregas.

The above objects and other objects and features of the invention willbe better understood from the following detailed description of severalembodimentsthereof taken together'with the drawings, in which:

FIG. 1 is a fragmentary section of the atmospheric furnace according tothe invention.

FIG. 2 is a fragmentarysection of a modified annospheric furnace of theinvention, and

FIG. 3 is a perspective view of a gas flow deflector shown in FIG. 1.

Referring to FIG. 1, the furnace constructed in accordance with theinvention is shown in a cross section through a plane which extendsvertically through the center of the fan and viewing toward thedischarge opening of an article being treated. While the fumace isillustrated as one used for batch operation, it should be understoodthat the invention is not limited to such furnace, but is equallyapplicable to a continuous furnace. Indeed, the illustrated furnaceportion may be taken as representing one zone such as a carburizing zoneof a continuous furnace. The furnace is generally shown at l and is ofconventional design in that it comprises a side wall la, ceiling lb andbase 1c, all of which are formed of refractory brick and which areexternally covered with a shaped body 2. and board 3 both formed ofconventional heat insulating material, with the outermost surface beinglined with a shell of steel sheet 4. A plurality of mounts 5 formed ofrefractory brick are arranged on the base 10 in an array along mutuallyperp'endicular axes, with a constant spacing between adjacent mounts,and are adapted to carry an article to be treated thereon within thespace 7 of the furnace. Specifically, skid rails, not shown in thedrawings are provided and guide a tray 16 on which is placed a basket 17mounting the article being treated.

The heat source of an atmospheric furnace usually comprises anelectrical heating element, which in the present example is shown at 6aand 6b positioned along the opposite side walls la and the base 1c ofthe furnace. The heating elements 6b are located intermediate adjacentrows or columns of the mounts 5. While the invention is not lirnitedtoany particular construction and lay-out of the electrical heatingelements, their arrangement shown is most customary and is known to bedesirable.

A fan 8 is mounted on the ceiling 1b so as to have its shaft 9 extendingtherethrough to be connected with an external electric motor 11. Arefractory seal 10 is provided around the shaft 9 between the shell 4and the motor housing.

In accordance with the invention, a gas inlet passage way 12 is formedaround and along the shaft 9 of the fan, and opens into the space 7 atits one end, while its other end is blocked by the refractory seal 10. Agas supply system 21 is connected with a guide pipe 13a including a gasseal 14a and which is connected with the inlet passage way 12. Thus, inthe atmospheric furnace shown, an atmosphere gas supplied from thesystem 21 is conducted through the guide pipe 131 into the inlet passageway 12, and thence to the fan 8 while turning around the shaft 9together therewith, and is diffused into the space 7 as a result ofrotation of the fan 8. The atmosphere gas at normal temperature has acooling effect on the fan 8 and the shaft 9 thereof, which effectprevents corrosion of the parts upon being heated to high temperaturesand results in reduced durability of these parts from occurring. Theatmosphere gas introduced into the furnace 7 through the gas inlet 12 isexhausted through suitably located outlets, which in the present exampleare shown at 15, adjacent to the discharge opening of the article beingtreated. As is known, the fluid resistance presented by the heatingelements 6 and the loaction of the gas outlets 15 must be deliberatelytaken into consideration when determining the gas flow, but in theatmospheric furnace of the invention, the uniform diffusion afforded bythe fan 8 causes the atmosphere gas to flow toward the base 1c generallyin a helical pattern. As a consequence, the speed of flow reduces as thebase 1c is approached,

whereby the degree of convection imparted to the heat second gas inlet20 is also connected with the gas sup-,

ply system 21 through a branch circuit which includes a flow ratecontrol valve 22, thereby allowing sufficient flow rate of theatmosphere gas to be supplied through the second gas inlet 20 tomaintain the thermal balance within the furnace. The second gas inlet 20may directly open into the space 7 of the furnace, but preferably isprovided with a gas flow deflector 18 at its inner end. As shown in FIG.3, the deflector 18 has a notch 19 opening in a particular direction.This enables cold gas to be directed to a desired location forrecovering the thermal balance within the furnace.

FIG. 2 shows a gas inlet arrangement useful in a carbonitriding furnacefor mixing more than one .gas therein. Like parts as in FIG. 1 arerepresented by corresponding reference characters. The referencecharacters 13b and 14b, not appearing in FIG. 1, represent an additionalguide pipe and an additional gas seal, respectively. Thus, thearrangement is essentially the same as in FIG. l,'except that a pair ofguide pipes 13a and 13b are provided, which are connected with the gasinlet passage way 12 for introducing different gases, for example,carburing gas and ammonia gas, into the furnace. As mentionedpreviously, the mixing of these gases at normal temperature involvesdifficulties because of formation of ammonium carbonate, which arereadily avoided by providing the common gas inlet 12 in the furnace.This is possible because the inlet passage way 12 can be maintainedabove the temperature at which formation of crystals of ammoniumcarbonate sphere gas introduced by said first gas inlet, said first gasinlet leading into a passageway formed along and around the shaft of thefan, and a second gas inlet located on the opposite side of the furnacefrom the first gas inlet, ,said second gas inlet introducing anatmosphere gas of normal temperature to cool the then relativelyelevated gas adjacent the second gas inlet to thus maintain an eventemperature of the gases within the furnace.

2. A gas atmospheric furnace according to. claim 1, further including aguide pipe connected with the passage way for conducting the atmospheregas from the first gas inlet therethrough to the passage way.

3. A gas atmospheric furnace according to claim 2,

further including another guide pipe, the two guide pipes conductingdifferent gases to the passage way.

4. A gas atmospheric furnace according to claim 1 in which said secondgas inlet is connected with a gas supply system through a flow ratecontrol valve.

5. A gas atmospheric fumace according to claim 1, I

further including a gas flow deflector located within the furnaceadjacent to said second gas inlet to direct the gases therein.

1. A gas atmospheric furnace for carburizing and carbonitridingtreatment of steel articles comprising a first gas inlet for introducingan atmosphere gas into the furnace under forced pressure, a fan forstirring the atmosphere gas introduced by said first gas inlet, saidfirst gas inlet leading into a passageway formed along and around theshaft of the fan, and a second gas inlet located on the opposite side ofthe furnace from the first gas inlet, ,said second gas inlet introducingan atmosphere gas of normal temperature to cool the then relativelyelevated gas adjacent the second gas inlet to thus maintain an eventemperature of the gases within the furnace.
 2. A gas atmosphericfurnace according to claim 1, further including a guide pipe connectedwith the passage way for conducting the atmosphere gas from the firstgas inlet therethrough to the passage way.
 3. A gas atmospheric furnaceaccording to claim 2, further including another guide pipe, the twoguide pipes conducting different gases to the passage way.
 4. A gasatmospheric furnace according to claim 1 in which said second gas inletis connected with a gas supply system through a flow rate control valve.5. A gas atmospheric furnace according to claim 1, further including agas flow deflector located within the furnace adjacent to said secondgas inlet to direct the gas entering said inlet to a desired location insaid furnace to recover or maintain the thermal balance of gasestherein.